Abstract
Non-adiabatic vibrational-electronic resonance in the excited electronic states of natural photosynthetic antennas drastically alters the adiabatic framework, in which electronic energy transfer has been conventionally studied, and suggests the possibility of exploiting non-adiabatic dynamics for directed energy transfer. Here, a generalized dimer model incorporates asymmetries between pigments, coupling to the environment, and the doubly excited state relevant for nonlinear spectroscopy. For this generalized dimer model, the vibrational tuning vector that drives energy transfer is derived and connected to decoherence between singly excited states. A correlation vector is connected to decoherence between the ground state and the doubly excited state. Optical decoherence between the ground and singly excited states involves linear combinations of the correlation and tuning vectors. Excitonic coupling modifies the tuning vector. The correlation and tuning vectors are not always orthogonal, and both can be asymmetric under pigment exchange, which affects energy transfer. For equal pigment vibrational frequencies, the nonadiabatic tuning vector becomes an anti-correlated delocalized linear combination of intramolecular vibrations of the two pigments, and the nonadiabatic energy transfer dynamics become separable. With exchange symmetry, the correlation and tuning vectors become delocalized intramolecular vibrations that are symmetric and antisymmetric under pigment exchange. Diabatic criteria for vibrational-excitonic resonance demonstrate that anti-correlated vibrations increase the range and speed of vibronically resonant energy transfer (the Golden Rule rate is a factor of 2 faster). A partial trace analysis shows that vibronic decoherence for a vibrational-excitonic resonance between two excitons is slower than their purely excitonic decoherence.
Highlights
The fast and efficient nature of electronic energy transfer from natural photosynthetic antennas to the reaction center1–3 has been an area of significant experimental and theoretical interest for several decades, the underlying physics is still controversial
We aim to answer questions such as: What vibrations are involved in energy transfer and the decoherence processes probed in 2D spectroscopy? Under what circumstances do vibrations become separable from the nonadiabatic dynamics? What relationships hold between these different decoherence processes? and How does vibrational-electronic resonance with delocalized intramolecular vibrations affect energy transfer?
The essential features of the dimer model developed in this study extend those in Ref. 22 and can be specified in a complete diabatic basis of localized vibrational and electronic states
Summary
The fast and efficient nature of electronic energy transfer from natural photosynthetic antennas to the reaction center has been an area of significant experimental and theoretical interest for several decades, the underlying physics is still controversial. Much of photosynthetic energy transfer takes place in Forster’s “intermediate coupling” regime, which has presented a modeling challenge.. Much of photosynthetic energy transfer takes place in Forster’s “intermediate coupling” regime, which has presented a modeling challenge.8,9 In this regime, a pioneering paper by Womick and Moran showed that vibrationalexcitonic resonance can allow energetic disorder to assist a)Present address: Department of Physics, University of Michigan, Ann Arbor, Michigan 48105, USA. The proposed explanation was that the antenna protein holding the pigments somehow preserves electronic coherence.11 Several investigations of these oscillations reported signatures consistent with hypotheses of purely electronic coherence between different excitons spanning more than one pigment and reversible energy flow between pigments and bath, termed quantum energy transport.. We proposed the fully alternative hypothesis that the longest-lived oscillations are purely vibrational beats from the ground electronic state of the antenna, with enhanced
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